We have used fluorescent and spin-labeled probes to investigate the binding of compound 48/80 to cell membranes. The fluorescent probe diphenylhexatriene has shown that mastocytoma cells, like some other forms of neoplastic cells, are more fluid in the hydrophobic region of the cell membrane than their normal counterparts. Fluorescent-labeled 48/80 binds to mastocytoma cells exhibiting a fluorescence polarization value which is similar to the calcium inophores X-537a and A-23187. Fluorescence quenching measurements indicate that the 48/80 binding site is close to that of stearic acid; however, unlike the fatty acid, the mobility of bound 48/80 is unaffected by detergents. Spin-labeled fatty acid probes have shown that the hydrophilic regions of mastocytomas and mast cell membranes are similar in fluidity. Inactive spin-labeled 48/80 polymers do not bind to mastocytoma cell membranes; however, the active polymers bind strongly to multiple sites which are inaccessible to the extracellular aqueous environment. Polymyxin B displaced 48/80 from these sites; however, morphine, curare and concanavalin A had no effect. Fluorescent and spin-labeled probes have been successfully adapted to the study of the action of compound 48/80 on the molecular level. BIBLIOGRAPHIC REFERENCE: Sinha, B. K., Cysyk, R.L., Millar, D.B. and Chignell, C.F.: Synthesis and biological properties of some spin-labeled 9-aminoacridines. J. Med. Chem. 19: 994-998, 1976.